A polymer film of typically cellulose ester, polyester, polycarbonate, cyclo-olefin polymer, vinyl polymer or polyimide is used in silver halide photographic materials, retardation plates, polarizers and image display devices. Films that are more excellent in point of the surface smoothness and the uniformity can be produced from these polymers, and the polymers are therefore widely employed for optical films.
Of those, cellulose ester films can be directly stuck to most popular polarizing films formed of polyvinyl alcohol (PVA)/iodine in on-line operation, because they have suitable moisture permeability. Accordingly, cellulose acylate, especially cellulose acetate is widely employed as a protective film for polarizers. The protective film for polarizers is demanded to be excellent in optical isotropy. In particular, the optical properties of a protective film disposed between the polarizing film and a liquid crystal cell have a large influence on the visibility of liquid crystal display devices.
On the other hand, with the recent broadening of the viewing angle, improvement of retardation compensation is desired, and the retardation of a film disposed between the polarizing film and a liquid crystal cell is desired not to change. That is, desired is a film having a small retardation independently of the viewing angle, in other words, a film having a small in-plane retardation (Re: this may be simply referred to as Re) and a thickness-direction retardation (Rth: this may be simply referred to as Rth); and these retardations are desired not to change independently of wavelengths and measurement environments. For conventional cellulose acetate films, however, there are such problems that the realization of a film having a small retardation independent of the viewing angle is difficult, and that Rth changes largely depending on the measurement environment, in particular, humidity environment.
Films formed of polycarbonate or cyclo-olefin polymer are proposed as the film having a small retardation independently of the viewing angle (e.g., see JP-A-2001-318233, JP-A-2002-328233). These films can be commercially available, for example, as ZEONOR (by ZEON) and ARTON (by JSR). But, since these films have low moisture permeability, they can not be stuck directly as a protective film for a polarizing film, as compared with a cellulose acetate film.
Accordingly, such film is desired that has a reasonable moisture permeability, and a small Re and Rth independently of wavelengths and measurement environments.
When transparent polymer films are applied to optical uses, for example, to optical compensatory films, supports for optical compensatory films, protective films for polarizers and liquid crystal display devices, the control of their optical anisotropy is an extremely important element in determining the performance (e.g., visibility) of display devices. Thus, controlling suitably the optical anisotropy of transparent polymer film is important. However, it is generally considered that the control of the in-plane retardation (Re) is easy, but that the control of the thickness-direction retardation (Rth) is difficult.
Since especially excellent optical films can be produced, a method of solution casting film formation is employed most commonly. In the method of solution casting film formation, however, since compressive force is added inevitably in the thickness direction in the film formation process, the production of the film having a low thickness-direction retardation is very difficult.
When optical films are produced by a method of melt film formation, too, force may be added to the web in the film-forming step or a transporting step, therefore the production of films having a low retardation is difficult.